In such tools, the rotary knife is set in motion by strokes of the second handle transmitted by the transmission means or mechanism and a workpiece inserted between the two cutting edges is severed as the clear span between the two cutting edges continously decreases. Up to now, both of the cutting edges, i.e. as well on the stationary cutting plate, as on the rotary knife, have been made exclusively concave, i.e. without any convex projections.
In order to achieve high cutting forces, mechanical drives such as toothed gears or toggle lever drives have been used hitherto as the force-and-motion transmitting means. Because of the high transmission ratio needed, and with regard to the mechanical strength of the components which can be achieved, toothed gears are less appropriate for a light, manually operated tool. As to toggle lever drives, the transmission ratio is again a limiting factor as to how great dimensions can be chosen, because the lever joints cannot be arranged arbitrarily close to one another, because with increasing transmission forces, increased diameters of the joint bolts call for an increased spacing and the transmission ratio is reduced. If the diameter of the rotary knife would be increased, a greater, heavier and more expensive tool would be obtained.